If she asks just play dumb, "What bike?.... Oh, I've had that one for a while......."
That's what I do and it seems to work. She'll scratch her head and say "Huh, I didn't notice it before." As long as it doesn't look to expensive she'll never think twice, If it is expensive I trade for it so I can say "Oh I traded some other bikes for it." Then she's happy cause I got rid of bikes and cut down the number of bikes I own and didn't spend any money doing it.